In recent years, semiconductor memories are used everywhere, for example, for the main memories on large computers, personal computers, electric appliances for home use, and mobile phones. Particularly, flash memories are getting most of the attention. Flash memories (Flash EEPROM (Electrically Erasable and Programmable Read Only Memory) type non-volatile memories) are used for many information units, such as mobile phones or digital cameras. This is because data do not disappear even if the power is OFF, and the memories have a configuration suitable for high integration.
The flash memories mainly include NOR and NAND flash memories. In the NOR flash memories, high-speed “Read” operations are possible. The number of possible “Read” times is approximately 1013. Thus, the NOR flash memories are used as instruction code memories of mobile units. However, the NOR flash memories are not suitable for file storage, because the Write bandwidth is narrowed.
On the other hand, the NAND flash memories can be more highly-integrated than the NOR flash memories. For “Read”, the access time is approximately 25 microseconds, so is very slow, but enables Burst Read operations. Further, the NAND flash memories have wide bandwidth. For “Write”, the “Program” (writing) time is approximately 200 microseconds, and “Erase” (deleting) time is approximately 1 ms, thus are very slow. However, a large number of bits can be “Programmed” or “Erased” at one time. Thus, a large number of bits can be “Programmed” in the unit of Pages at one time. Accordingly, the NAND flash memories can be highly-integrated, have wide bandwidth, and thus are used for memory cards, USB (Universal Serial Bus) memories, mobile phones, memories for portable music players. Recently, SSD (Solid State Drive) is for sale as replacement for the hard disk.